The present invention relates to a strobo flash device of the type in which the quantity of strobo light may be automatically controlled for the optimum flash exposure.
In the conventional automatic strobo flash devices of the type disclosed in Japanese Patent Publication No. 30,905/1969 l and U.S. Pat. No. 3,591,829, a strobo tube is connected in series to a first current control element which, in turn, is connected in parallel to a control circuit consisting of a switching capacitor and a second current control element. A photometric circuit is provided in order to detect the quantity of light reflected back from a subject. When the quantity of reflected light reaches a predetermined level, the photometric signal generates a signal in response to which the second current control element is conducted to discharge the switching capacitor so that the first control element is turned off to extinguish the flash tube. Therefore, a part of the energy stored on a main flash capacitor is converted into light depending upon the distance to a subject, so that the loss of the energy of a power supply may be prevented. Moreover, when the distance to a subject is relatively shorter, the flash interval or time between flashes may be reduced so that the successive flash exposures may be effected at a shorter interval without missing a chance.
When the stobo tube is lighted, the large current of the order of hundreds ampers flows through the first current control element, so that the switching capacitor must be accordingly so large as to flow the large reverse current to turn off the first current control element. It takes some time before the switching capacitor is recharged to a level sufficiently high to turn off the first current control element. When the strobo tube is lighted before the switching capacitor has not reached yet this level, the first current control element cannot be turned off by the discharge of the switching capacitor even when the second current control element is turned on in response to the output signal from the photometric circuit. As a consequence, the desired automatic control of the quantity of light cannot be attained. The flash duration is extremely short and is of the units of millisecond, so that it is impossible to detect by vision whether or not the automatic light quantity control has been successfully accomplished.